The present invention relates to atomic absorption spectroscopy photometers and more particularly to a multi-element simultaneous analysis atomic absorption spectroscopy photometer which analyzes a plurality of elements simultaneously.
A multi-element simultaneous analysis atomic absorption spectroscopy photometer simultaneously impinges light beams from a plurality of hollow cathode lamps (light sources) including the bright lines of elements to be detected on a heating furnace (sample atomizing unit) at a fixed angle to the furnace, selects the absorption wavelengths of the light beams absorbed by the atomic vapor of a sample which occurs during heating using a spectroscope provided after the heating furnace, detects the optical intensities of the selected wavelengths using photodetector systems, and determines the multiple elements contained in the sample from the proportions of absorption by those elements. The same applicant has proposed a photometer for analyzing multiple elements simultaneously, as disclosed in Unexamined Japanese Patent Publication JP-A-63-292040.
However, the proposed photometer has the following drawbacks:
(1) The conventional photometers disposed in the corresponding optical systems each have a single incident slit having a fixed width and a single exit slit having a fixed width, so that they each may not be suitable for an element to be measured and thus the sensitivity of the spectroscope is likely to decrease depending on that element. As the slit width becomes wider, a greater quantity of light is usually obtained, so that a higher S/N ratio signal is obtained and an electric current which lights a lamp is reduced, and the lamp service life is increased advantageously. However, as the slit width increases, a so-called atomic absorption wavelength which absorbs light most efficiently cannot be separated from a neighboring absorption wavelength (neighboring line) close thereto and the absorption sensitivity can decrease depending on an element to be measured, due to the background produced by the neighboring line. FIG. 8 illustrates the state of neighboring lines of an iron cathode lamp, and FIG. 9 illustrates an extraction of some examples of atomic absorption wavelengths of elements, the presence/absence of neighboring lines and recommended slit widths to avoid those neighboring lines. The closer the neighboring line is, the narrower the slit width should be. Since elements such as arsenic having a low melting point have low absorption efficiency, the slit width is required to increase sufficiently. FIG. 10 illustrates the effect of a slit width on an iron working curve having a neighboring line. It will be seen that as the slit width becomes narrower, the absorbency for the same density increases and the sensitivity also increases. As illustrated by the above examples, the sensitivity for some elements decreased when the spectroscope used had a single slit and the entire optical system had the same fixed slit width. For example, among the elements illustrated in Table 9, it was difficult to cope with a combination of iron and nickel for which it is desirable to reduce the slit width because they produce a neighboring line, and arsenic and selenium for which it is desirable to increase the slit width because the lamp used is dark.
(2) Although the respective diffraction gratings of a spectroscope are disposed independently in the corresponding optical systems, they have exactly the same specifications. Therefore, they do not match with the atomic absorption wavelength regions of some elements and do not provide sufficient sensitivities. When the reflective surface constituted by the angle of a roof-like groove constituting a diffraction grating, or a so-called blaze angle, properly faces parallel incident light beams thereon in a Littrow grating spectrograph or when a similar refractive surface of a Czerny-Turner spectrometer is between parallel incident light beams and a dispersive reflective angle, the reflective efficiency becomes maximum on the diffraction grating surface, in which this wavelength is called the "blaze wavelength". FIG. 11 illustrates how the relationship between the diffraction light efficiency and wavelength of a diffraction grating varies with blaze wavelength. If a diffraction grating having a blaze wavelength more suitable for a wavelength or element to be measured can be selected, the sensitivity will be increased greatly.
(3) Also, although photomultipliers each comprising a detector are disposed independently in the corresponding optical systems, they are the same in specifications. Therefore, they have not necessarily suitable for the atomic absorption/wavelengths of some elements to be measured and do not provide enough sensitivity. FIG. 12 shows the sensitivity vs. wavelength characteristics of photomultipliers. The characteristic of the photomultiplier varies depending on the kind of the materials constituting its photoelectric face. All the detectors are fixed to cover the same wavelength range of 190-860 nm serving as an atomic absorption spectroscopy photometer. About half of all the elements have an atomic absorption wavelength of less than 250 nm. If a photomultiplier having a sensitivity characteristic more suitable for a wavelength or element to be measured can be selected, the sensitivity will be improved greatly.
As just described above, although the conventional atomic absorption spectroscopy photometers for multi-element simultaneous analysis have various optimal device conditions such as the slit widths of the spectroscope, the blaze wavelengths of the diffraction gratings, the wavelength characteristics of the detectors, etc., due to atomic absorption wavelengths, they have not taken those conditions into consideration and hence have not derived sufficient device performances.